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# Java Recursion

In Java, a method that calls itself is known as a recursive method. And, this process is known as recursion.

A physical world example would be to place two parallel mirrors facing each other. Any object in between them would be reflected recursively.

### How Recursion works?

In the above example, we have called the `recurse()` method from inside the `main` method. (normal method call). And, inside the recurse() method, we are again calling the same recurse method. This is a recursive call.

In order to stop the recursive call, we need to provide some conditions inside the method. Otherwise, the method will be called infinitely.

Hence, we use the if…else statement (or similar approach) to terminate the recursive call inside the method.

## Example: Factorial of a Number Using Recursion

``````
class Factorial{

static int factorial( int n ){
if (n != 0)  // termination condition
return n * factorial(n-1); // recursive call
else
return 1;
}

public static void main(String[] args){
int number = 4, result;
result = factorial(number);
System.out.println(number + " factorial = " + result);
}
}``````

Output:

`4 factorial = 24`

In the above example, we have a method named `factorial()`. The `factorial()` is called from the `main()` method. with the number variable passed as an argument.

Here, notice the statement,

``return n * factorial(n-1);``

The `factorial()` method is calling itself. Initially, the value of n is 4 inside `factorial()`. During the next recursive call, 3 is passed to the `factorial()` method. This process continues until n is equal to 0.

When n is equal to 0, the `if` statement returns false hence 1 is returned. Finally, the accumulated result is passed to the `main()` method.

## Working of Factorial Program

The image below will give you a better idea of how the factorial program is executed using recursion.

When a recursive call is made, new storage locations for variables are allocated on the stack. As, each recursive call returns, the old variables and parameters are removed from the stack. Hence, recursion generally uses more memory and is generally slow.

On the other hand, a recursive solution is much simpler and takes less time to write, debug and maintain.

## Java tutorials for Beginners – Java Recursion

#### Free Machine Learning & Data Science Coding Tutorials in Python & R for Beginners. Subscribe @ Western Australian Center for Applied Machine Learning & Data Science. ```